Means for receiving radio signals



May 4, 1943 E. H. ARMSTRONG MEANS FOR RECEIVING RADIO SIGNALS Filed Jan. l2, 1940 Patented May 4, 1,943

n UNITED STATES PATENTol-Fica MEANS FOR RECEIVING RADIO SIGNALS Edwin H. Armstrong, New York, N. Y. Application January 12, 1940, Serial No. 313,495

4 Claims. (Cl. Z50-20) This invention relates to an improvement in receiving apparatus for frequency modulated waves. It has for its object the provision of means for automatically adjusting the Width of the admittance band of the sharp circuit in receivers of the type described in my U. S. Patent No. 2,116,502.

Referring now to the gures which form a part of this specication, Fig. I illustratesthe general arrangement of the apparatus for carrying out the inventin. Fig. l1 shows a. series of characteristic curves which will be referred to hereinafter.

The general theory of the invention will be understood from the following explanation. In my U. S.*Patent No. 2,116,502 I have pointed out how the effects of certain types of disturbances may be reduced in areceiver for a frequency modulated wave whose deviation is considerably greater than the frequency range of audibility` by including in the filtering system a circuitzrwhose band width is considerably narrower than the normal width of admittance. With this arrangement itis possible to produce at the current limiter a greater ratio of signal carrier to those disturbing currents resulting from ignition noises and the like, particularly during those periods when the transmission is not heavily modulated.

It has been found by experiment that the band width of the sharp circuit which is determined by the amount of resistance introduced into itis not a constant quantity for all values of signal to noise ratios. At its narrowest point, it is found that the Width of the band is at least twice the frequency of lthe modulating current so that curtailment of the signal frequencies and resulting distortion is avoided. In general for strong signais the band widthof this ,selective circ be made wide whilefor weaker-fs results are obtained with a narrowe y l introduces an additional adjustment "whfic itf desirable to avoid. v

It is the purpose of this invention to provide la' means by which the band Width is automatically adjusted by means of the signal-level itself. The manner in which this is accomplished may best be understood by reference to Fig. I. Referring u may bination for heterodyning the first 'intermediate frequency current down to a second intermediate frequency value. 9 represents an amplifier for this second intermediate frequency value. will be understood that these intermediate frequency amplifiers have inherently selective coupoint of the second intermediate frequency. The

voltage developed across this circuit is impressed on the grid of another amplifier I5 through a blocking capacity I3. A resistance I 4 as shown acts both as a grid leak and as a damping resistance on the circuit II, I2. This resistance will be referred to hereinafter. The amplifier I5 drives the current limiter I6 in the usual manner and the outputof the limiter is passed through a filter II torer'nove harmonics, a converter I8 to change the frequency changes inthe current into amplitude changes, and a detector I9 wherein the original currents of the signal are reproduced. Now it will be observed that the tuned circuit II, I2 is placed in a tube circuit just preceding the current limiter. The level is, therefore, quite high as it ordinarily requires ten to twenty volts on the grid of the limiter for its proper operation. By adjusting the amplification of the tube I5 the voltage required on its grid to give sufficient output to make the current limiter operate properly may be made just less than would cause grid current to flow through the resistance I4.' IUnder these conditions the widthof the resonance curve of the circuit II, I2 is determined solely by the value of the resistance whi'clr'inaaty, be set at the best value for a weak jgnal QfWhen, however, a stronger signal is re- 1 ceived. the higher voltage developed across the circuit II,` I2 causes the tube I5 to draw grid I current. The effect of this is to introduce adnow to Fig. I, there is illustrated a receiver of the f type described in my U. S. Patent No. 1,941,069.

I-2 `represents the antenna, 3 an amplier for the received currents, 4 5 a rectifier and oscillator for heterodyning down the frequency of this current to a suitable intermediate frequency value, 6 an amplifier for this intermediate frequency and 'I--8 a second rectifier oscillator com..

ditional resistance into the circuit II, I2 and to therefore broaden out its resonance curve. Fig. II shows a series of resonance curves of circuit II, I2 for various voltages impressed upon the grid of the amplifier I0.

It will -be seen, therefore, that as the signal decreases in strength, the width of the admittance band of the tuned circuit may =be made to close up to practically any desired value by the variation in the grid current drawn in the tube .which is'connected across the tuned circuit. By

Itv

. properv proportioning of the various values yvolved substantially any law of response may be obtained.

I have described what I believe to lbe the best embodiment of my invention. I dq not wish, however. to .be confined to the embodiment shown, but what I desire to cover lby Letters Patent is set forth in the appended claims:

1. A radio receiver for freuqency modulated signaling currents comprising. in combination, a resonant 4circuit adapted to pass a band offrequency modulated signaling currents, and control means for automatically increasing the pass band of the resonant circuit as the circulating current L in the resonant circuit increases, said control means comprising a vacuum tubearranged to amplify the current in the resonant circuit, said tube having a grid and cathode and an input circuit conductive to direct current connecting applying a negative bias voltage to the grid of the tube, means coupling said resonant circuit and said grid circuit. and means whereby the voltage developed across said resonant circuit upon an increase in the strength of the received signal causes grid current to now in said grid circuit whereby to load the resonant circuit and automatically increase its band width as the strength of the signal increases. v

3. The combination with a radio receiver for wide bandfrequency modulated signaling currents, of a selective resonant circuit interposed in said receiver and adapted to pass a band of the received frequency modulated signaling currents and means responsive to the strength of the said frequency modulated currents, but substantially unaffected by the extent of the frequency deviations thereof, for changing the effective pass band of the selective circuit directly in accordance with the strength of the said frequency modulated signaling currents.

4. A radio receiver for frequency modulated signaling currents comprising, in combination, a selective resonant circuit tuned to a predetermined center frequency and adapted to pass a -band of frequency modulated signaling currents and energy consuming means connected across the terminals of the resonant circuit and directly responsive to the amplitude of the .circulating currentl in said circuit, but. substantially unaffected by the extent of the frequency deviations thereof, for automatically loading the circuit,

`whereby the band Width of the resonant circuit automatically increases as the strength of the received signal increases.

EDWIN H. ARMSTRONG. 

